In order to make integrated optical devices as semiconductor micro lasers, one has to be able to guide the light through the device. This can be accomplished by making optical waveguides in the device, as ridge waveguides or alike. For a junction laser, such a ridge can be straight, bended and/or with junctions. For junction lasers such as a Y-junction, the device performance of the laser is dependent on the resolution at which the junction can be etched. Improved resolution means a more V-like shape of the inner part and better power transmission as the junction becomes a true Y-junction (P. Sewell et al. (1997)). This has traditionally been carried out by dry etching techniques as reactive ion etching (RIE) (K. Al Hemyari et al. (1993)). The RIE process can result in an isotropic etch in which the surface is etched normal to the surface plane, and sidewalls of the ridge features are straight.
GaSb-based materials as AlGaAsSb and InGaAsSb has previously been used to make lasers (Choi et al. (1991), Simanowski et al. (2001), Yarekham et al. (2000)). These lasers have all been high-power multimode lasers with applications as remote sensing and alike. This is not good enough for all electro optical microsystems in which optical single mode operation is sometimes required.
A wet etching process had been previously developed (R. Bugge et al. (2002)) which could etch AlGaInAsSb materials with good control and anisotropic features. For the patterning in the present invention, this etchant was used to make the new structures.
A new process for increasing the doping concentration of Te-doped material has been previously submitted in a separate patent (R. Bugge (2004)).
A new penternary semiconductor structure based on three or more layers of AlInGaAsSb is proposed for making single mode optical components. The structure can be used for both active or passive components and specifically mid-IR lasers. By using AlInGaAsSb material in the cladding and spin-etch processing, good etch-control has been achieved. Accurate ridge waveguides has been fabricated with this wet etch method. The ridge waveguides gives single mode operation when designed and made with predetermined refractive indexes in core and claddings. To compensate for refractive index changes in cladding when increasing In-content, AlInGaAsSb material can also be used in the core.
For active devices, a MQW active region of AlGaAsSb or AlInGaAsSb barriers and InGaAsSb or AlInGaAsSb wells, was embedded in the core. In was used in the barriers for prevention of band edge bending between the core and barrier (when the core contained In). The addition of small amounts of Al to the InGaAsSb wells will increase the band gap of the well. Due to In-diffusion, the MBE growth temperature of the substrate has to be kept below 490 C once InGaAsSb material has been grown on it, as compared to 550 C for AlGaAsSb. During sequential growth of AlGaAsSb or AlInGaAsSb on top of the AlInGaAsSb, a higher growth temperature than the 490 C limit (for InGaAsSb) can be used due to smaller net In-diffusion between the wells and barrier. Another advantage with this structure is that Rapid Thermal Annealing (RTA) can later be carried out on the structure without any significant In-seggregation (In-diffusion from the wells into the barriers).
In order to make junction lasers and other optical junction devices with wet etching, the device design has to be altered from the traditional junction design. In wet etching, the V-shape of a Y-junction will end up as an U-like shape after processing due to the anisotropy of the etch. For a Y-junction designed device, this would result in a non-working device, but using the design rules for a device in the present invention, a working junction based device can be made.
To make a ridge on a wafer, one has to apply a masking material to the surface of the wafer. After processing, the masking material will define the pattern of the ridge. During sequent processing of the wafer, a wet chemical etchant will etch the material which is not masked by the masking material. Due to the anisotropy of the wet etching (used here), the etching will result in some etch beneath the masking material at its edges (underetch), as shown in FIG. 1. In this figure we see a ridge of the semiconductor material with photoresist masking at its top. During sequential processing, the photoresist will be removed and the result is a free-standing ridge. This underetch had to be taken into consideration when we designed the ridge, as it results in a U-like feature at the inner part of the junction as shown in FIG. 2. Such a U-like feature will result in loss of light in a traditional Y-junction ridge structure.
The idea of the present invention was to incorporate bends in the opposite direction of the junction bends to prolong the waveguide in the junction area and collect any light which is lost in the U-shaped feature of the Ψ-Junction (see FIG. 3). The U-shaped feature is a result of the isotropic wet etching, so that reducing the importance of this feature was important to make useable wet etched junctions.
During design, the optical waveguiding properties of the Ψ-Junction based device had to be simulated in order to test the junction prior to making it. Using the beam propagation method (BPM), we simulated ridge waveguide junctions. FIG. 5 shows the propagated optical field in one of our Ψ-Junction designs. Some splitting loss can be seen, although most of the field remains in the waveguides.
Optical coupling of the devices in the current invention, is done by coupling the waveguide ends to other wave guiding devices through optical fibers, embedded waveguides, planar waveguides, ridge waveguides, tenses and alike. By using coatings of higher or lower reflection, or designs that adjust the optical field at the waveguide ends of the device, coupling loss can be reduced.